The present invention relates generally to the thermal spraying of powdered materials, as well as their application to surfaces as protective coatings.
A variety of thermal spray coatings have long been used to protect various components. A principal variety of thermal spray coatings to which the subject matter of the present invention pertains is the plasma spray process, This process has been used to apply many different types of coatings in numerous industries.
For the most demanding of service conditions (e.g., the high pressure turbine section of a modern gas turbine engine), protective coatings of the MCrAlY and thermal barrier type are often used. MCrAlY materials are generally comprised of a base metal M (including Ni, Co, Fe, and mixtures of these elements), Cr, Al and Y. Modifications of these coatings have included additions of other materials such as Si, Ta, Hf, and others, to enhance the resistance of such materials to high temperature oxidation and to improve mechanical properties.
Such coatings are conventionally applied either as a single layer (McrAlY) coating or as a dual layer coating (McrAlY layer and a ceramic layer). Protection of the surface of the component (i.e., the substrate) that receives such coatings is provided by various metallic constituents present in the coating. Adhesion of the coating to the surface of the component is accomplished by a thin interdiffusion layer that is formed after a post-coating heat treatment of the applied materials.
Another use for McrAlY coatings is as an essential part of a thermal barrier coating (TBC). Generally speaking, a thermal barrier coating is a multi-layer coating system that includes an insulating ceramic outer layer known as a "top coat", and a metallic inner layer known as a "bond coat". The bond coat is located between the top coat and the substrate which is to receive the thermal barrier coating. To a large extent, the durability of a thermal barrier coating depends upon the durability of the intermediate bond coat, since this layer serves to prevent separation of the thermal barrier coating from the substrate which receives it. The durability of the bond coat primarily depends upon three factors including the chemical composition of the bond coat, the process used to apply the bond coat to the substrate, and the surface finish of the substrate which is to receive the thermal barrier coating.
It has been found that McrAlY coatings used as overlay coatings are suitable as the bond coat for a thermal barrier coating. These McrAlY coatings can be applied to the substrate using a variety of thermal spray processes. Among the most popular of these are air plasma spraying (APS), argon-shrouded plasma spraying, vacuum plasma spraying and high velocity oxyfuel spraying (HVOF). In connection with the application of such materials to a substrate as a bond coat, more durable McrAlY coatings have been sought. Moreover, for use as a bond coat, a certain minimum roughness was found to be necessary for the ceramic top coat to mechanically adhere to the bond coat. Processes that would yield a coating with a minimum porosity and minimum amounts of oxides were also found to be desirable.
Although air plasma spraying is a relatively inexpensive process, the resulting coatings are characterized by significant amounts of porosity and oxides in the resulting (McrAlY) coating. Conversely, vacuum plasma spraying leads to high quality coatings, but at a very high cost. Argon-shrouded plasma spraying has also been found to produce an effective coating, but requires an injection of a secondary gas at a very high flow rate. High velocity oxyfuel spraying cannot easily provide sufficient heating of the powdered materials to achieve an acceptable coating. As a result, high quality coatings at a relatively inexpensive cost have generally not been attainable.
Similarly, for use as a bond coat in a two layer plasma sprayed thermal barrier coating, both roughness and purity are critical to achieving a satisfactory useful life. To this end, the generally accepted method for achieving a high quality bond coat is to spray the desired powder in a vacuum chamber. Again, this leads to certain disadvantages in actual implementation.